Nuclear Magnetic Resonance (NMR) Imaging, or Magnetic Resonance Imaging (MRI) as it is commonly known, is a non-invasive imaging modality that can produce high resolution, high contrast images of the interior of the human body. MRI involves the interrogation of the nuclear magnetic moments of a subject placed in a strong magnetic field with radio frequency (RF) magnetic fields. An MRI system typically comprises a fixed magnet to create the main strong magnetic field, a gradient coil assembly to permit spatial encoding of signal information, a variety of RF resonators or RF coils as they are commonly known, to transmit RF energy to, and receive signals emanating back from, the subject being imaged, and a computer to control overall MRI system operation and create images from the signal information obtained.
The large majority of RF coils used in MR imaging are tuned to 1H due to the high abundance of this paramagnetic nucleus in the body, and the resulting ability to produce detailed structural images of body and tissue structure. However, several alternate paramagnetic nuclei are desirable for MR imaging and spectroscopy applications such as for example 13C, 31P, 23Na, etc. . . . . Each of these applications however, requires a special RF coil tuned specifically to the particular resonant frequency of the selected paramagnetic nucleus.
In many MR imaging and spectroscopy applications it is desirable to simultaneously or sequentially probe multiple paramagnetic nuclei. Using multiple single tuned RF coils for the various paramagnetic nuclei is inconvenient, requires time consuming RF coil changes, and patient repositioning, which can result in registration errors. In addition, investigations utilizing the Nuclear Overhauser Effect (NOE) or Proton Decoupling are not possible with single tuned RF coils. To deal with these problems, RF coils capable of multiple tuning for MR imaging applications have been considered.
For example, U.S. Pat. No. 6,236,206 to Hartman discloses a birdcage coil capable of multiple tunings for different paramagnetic nuclei. Unfortunately the birdcage coil can be tuned only to one resonant frequency at any one time limiting the birdcage coil to single tuned imaging applications.
U.S. Pat. No. 4,916,418 to Rath discloses a double tuned birdcage coil in which discrete inductors are placed across the leg capacitors to introduce a second set of resonances. Unfortunately, the birdcage coil is not of an interleaved coil design and is not particularly suited to very high-field (3 Tesla (T) or greater), large volume imaging applications.
U.S. Pat. No. 5,202,635 to Srinivassan discloses a dual tuned RF coil based on a four (4) ring low-pass birdcage coil. Similar to the Rath birdcage coil, this birdcage coil is not an interleaved structure and therefore, results in different sensitive volumes for the H+ and the alternate ‘X-nuclei’ modes, which can lead to registration errors. Also as the birdcage coil is based on a low-pass design, it suffers significant electric field losses making it unsuitable for very high-field (3 T or greater) imaging applications.
U.S. Pat. No. 5,680,047 to Srinivassan discloses a dual tuned coil based on a solenoid coil geometry. As the coil does not conform to a minimum inductance design, it suffers electric field losses making it unsuitable for very high-field imaging applications.
U.S. Pat. No. 5,144,240 to Mehdizadeh discloses a dual tuned birdcage coil in which discrete inductors are added across the leg capacitors to introduce a second set of resonances. Similar to the Rath birdcage coil, this birdcage coil is not of an interleaved coil design and is not particularly suited to very high-field imaging applications.
U.S. Pat. No. 6,100,694 to Wong discloses a multiple tuned birdcage coil based on a four (4) ring birdcage design. As the birdcage coil is not an interleaved coil structure, it results in different sensitive volumes for the H+ and X-nuclei modes, which can lead to registration errors. Also as the birdcage coil is based on a low-pass design, it suffers significant electric field losses making it unsuitable for very high-field imaging applications.
U.S. Pat. No. 5,041,790 to Tropp discloses a dual tuned RF coil in which separate coils are used and tuned to different paramagnetic nuclei of interest. The RF coil uses a birdcage coil for H+ imaging and Helmholtz coils for alternate paramagnetic nuclei.
U.S. Pat. No. 5,194,811 to Murphy-Boesch discloses a dual tuned RF coil design based on a four (4) ring low-pass birdcage coil. Similar to a number of the above-described designs, this birdcage coil is not an interleaved structure and therefore, results in different sensitive volumes for the H+ and X-nuclei modes, which can lead to registration errors. Also as it is based on a low-pass design, the birdcage coil suffers significant electric field losses making it unsuitable for very high-field imaging applications.
The publication entitled “A Multiply-Tuned Hybrid Birdcage Volume Resonator for Transmit/Receive and Transmit-Only High Field NMR Imaging, Spectroscopy, and Multi-Nuclear Phased Array Applications” authored by Barberi. et al. and published in the proceedings of the International Society of Magnetic Resonance Medicine in 2002 discloses a dual tuned resonator comprising a sixteen (16) element cylindrical hybrid birdcage resonator. Reactive tuning elements are placed on the resonator end rings and column elements. Tuning the resonant structure to two (2) distinct, homogeneous modes is achieved using a combination of single-valued end ring capacitances and an arrangement of distinct-valued, alternating, interleaved impedance elements on the columns. The discrete column impedance elements may be capacitive, zero impedance or inductive. Alternating capacitive elements on the columns effectively yields two interleaved hybrid resonators. Alternating capacitive and zero impedance elements effectively yields a high gamma hybrid birdcage resonator interleaved with a low gamma high pass birdcage resonator. The two distinct homogeneous modes of the resonant structure are isolated from each other. Isolation is provided through the placement of discrete element series/parallel resonant pass-reject electronic blocking networks on the low gamma and high gamma sections of the volume resonator.
The electronic blocking networks associated with the high gamma section are passive electronic high gamma pass/low gamma reject networks while the electronic blocking networks associated with the low gamma section are passive electronic high gamma reject/low gamma pass networks. Each of these networks comprises three discrete reactive components. The networks are used to maintain the original resonant frequency of either the low gamma section or high gamma section when either is tuned in the absence of other gamma tuning components. This attempts to ensure coupling isolation between the high gamma section and the low gamma section of the volume resonator.
Unfortunately, this volume resonator suffers disadvantages in all of the electronic blocking networks for isolating the low gamma and high gamma sections of the volume resonator must be precisely aligned in order to avoid sensitivity degradation. As will be appreciated, aligning the blocking networks is an extremely difficult task. Also, as the volume resonator makes use of an inductive drive to preserve symmetry of the volume resonator, the volume resonator requires additional adjustments to achieve desired coupling and preserve resonator homogeneity. This is due to the fact that the inductive drive loops produce their own magnetic fields that perturb resonator homogeneity. Furthermore, the volume resonator requires a shield to reduce radiative losses at the high gamma frequency.
As will be appreciated, there exists a need for a volume resonator capable of simultaneous or sequential probing of multiple paramagnetic nuclei (‘multiply-tuned’, e.g. 1H for structural information and 13C or 31P for spectroscopy data) where each paramagnetic nuclei has the same sampling volume within the subject being imaged. There also exists a need for a multiply-tuned volume resonator which is particularly suited to high-field MR imaging systems and which can be unshielded for non-claustrophobic clinical applications.
It is therefore an object of the present invention to provide a novel multiply-tuned volume resonator for magnetic resonance imaging and spectroscopy.